I've heard between 30-40mph, but I am pretty sure I have hit 45 and felt safe and in solid control of my bike. If you look down and see you are above XXmph, you decide it is time to put on the brakes...what is that speed?

Edit: In summary, how can one determine what a safe speed is for a particular bike in a particular situation?

I'd like to see this tidied up, but I'm not quite sure how. Perhaps close it and ask a new question more along the lines of "how can I safely ride faster" or "what techniques will help me stay on the bike at high speed"?
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МסžMay 2 '11 at 2:59

the answers below are the kind of responses i was seeking with this question, so i don't care to rephrase it..the intent of the question has been properly conveyed.
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ditojimMay 3 '11 at 16:24

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"For example, if you look down and see you are above XXmph, you decide it is time to put on the brakes...what is that speed?" This is totally subjective and not a real question.
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user313May 23 '11 at 22:28

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I agree that asking for a number is quite subjective, and I've downvoted for that reason and that the question is an invitation to discuss the issue, not the point of SE sites. However, the main point of this question is "how to determine what a safe speed is for a particular bike in a particular situation". The answers below have answered that, and I think having those answers on the site is valuable. Have done an edit that may help.
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Neil FeinMay 24 '11 at 0:59

I think that the answer to this question is not "how fast can you safely go?", but rather to look at the inverse, "how fast do you need to safely stop?". If you are on a long road with no side roads, and no chance of animals, dirt, gravel, stopped cars, braking cars, etc, then your maximum speed is very much a personal decision. As others in this question have mentioned, there are others who have almost certainly done it faster!

However, in the real world where you do need to deal with some of these things… coming off of your bike at 40mph, is going to hurt… at best. Braking at those speeds is also going to be challenging, and, just like in a car, the faster you are going, the longer it takes to brake. And this is not a linear relationship. Add to that your brakes won't work as well anyway, since you also need to fight gravity when you are on a hill.

On one of my bikes with an analog speedometer, back when I was in really good shape, I decided to find out. Got a good run onto the biggest downhill around and hammered it all the way down. Got the speedo up to an indicated 60mph before running out of hill (65 is where it maxed out, not sure how accurate it was). It was very smooth feeling, all the low speed vibration and bumps pretty much went away, however, I realized that if anything bad were to happen that I was going to die. Never again.

All the answers referring to safe stopping are exactly right, but then there is another consideration - your equipment. @moz uses m/s and this gives a real feeling for the amount of distance you can cover in the time it will require to observe, decide and then act.

Add to that your equipment. I remember a friend once discussing their recent Alpine trip and his first major experience of serious mountains. He'd been doing a lot of climbing training in the lead up and knew every decent ascent within 30-40 miles of home, but the biggest thing that you couldn't train for without the same conditions were the descents. When gravity and the road is making you go faster than you are comfortable with - and when the friction of your brake blocks heat the rims so much your tubes explode and then the blocks melt and solidify against whatever they touch.

A maximum safe speed is basically how fast can the rider stop given the ambient conditions and their equipment. There's never going to be one answer for all situations, but the faster you go, the better you're going to need to be - and the better your kit will need to be, too.

Look at the downhill sections of the Tour for example, where the riders are doing 30m/s or more. Or gravity bike racing, where the offroad riders regularly hit 30m/s (60mph) and the record is 130mph, and interestingly the on-road riders are slower, probably because of a lack of suitably steep paved roads. As a comparison point, the UCI record for an upright bicycle is about 20 m/s (45mph).

Personally I don't go much above 15m/s on my uprights and never above 20m/s. But with more wheels and a safer riding position I've felt fairly safe doing a 10 minute descent between 20 and 30m/s. I don't know what my maximum speed was there as my speedo loses the peak speed at ~10m/s and gives a maximum of 146km/hr (presumably related to wheel size and minimum possible time interval on the sensor).

I think that even if you feel confident & safe going at a higher speed this is a dangerous thing to do. You can never tell if a car is going to swerve into your path, if a cow might stroll out of a bush or if there's a pothole underneath that seemingly shallow puddle just waiting to throw you right off your bike.

If you go at very fast speeds you have to take into consideration you might fall. If you're travelling at 50mph and fall you're in some fairly serious trouble.

I have hit 50 mph on a Calfee road bike, no problem. Always felt more secure on my tandem though.
I spin out the crank at about 40 to 45 mph with 53x11 depending on my level of training.
I stopped going that fast though as I am too aware of the consequences of a fall.

On a finely-tuned road machine, running at 50+ mph is readily accessible and "feels" just fine on a smooth road. Probably would feel solid at twice that speed. But it isn't really a question of how fast the bike can go. It's a question of how you're going to feel when you eventually encounter that inevitable, out-of-the-blue, un-anticipated stop one day. Bang.

I recently had the pleasure of going over the handlebars at 16 mph on a downhill single track. Result was a Bell helmet in 8 fragments after landing on my head, a fractured vertebrae, collarbone dislocation, and broken finger. Takes surprisingly little speed to do serious damage.

I've enjoyed my share of road descents pushing 50 mph. But make no mistake about it: at these speeds you're almost certainly risking death if something goes terribly wrong.

Two questions, actually... How fast can you go, and how fast can you go safely...

Pro road racers have top-flight equipment checked on a daily basis and great skills as well...But even in that elite group there are descenders who can hammer and those who sort of cruise down the big descents..
As well, those are closed roads with no traffic guaranteed.

As for us mortal men... Consider traffic, what shape your bike is in, etc. I've gone 45+, and felt solid, but more than that would be pushing.
My now-deceased buddy said he passed a VW bug on a mountain descent out in Colorado once...

Factors beyond your control like inattentive motorists or other cyclists not expecting a fast bike

Legal speed limits

I recently topped out at 49mph going down a hill on a loaded touring bike. During the entire run, I was continually thinking "what if..." What if I hit a stick or pothole? What if a car pulls onto the road? What if my wheels start to wobble? If you can't answer the questions, slow down.

I also learned something about wheels at high speed. On another downhill run, I hit a pothole and my wheel was jarred out of balance. The entire bike began shaking and i had to execute an emergency stop. Inspection revealed no damage and when I continued downhill at a slower speed, the wheel was perfectly true again. If a wheel is momentarily knocked out-of-true at high speed, centrifugal force will hold the wheel out of balance until you slow down and then will often return to its original dimensions.

Considering the recent death on a descent at the Tour of Italy, the consequences of misjudging the safe speed are severe.

Best to go rather slow.

I try to keep my descending speed below 20mph on public roads. (Yes, I can do 40+ on those descents, but there are side streets and driveways and some debris. Stopping distances are longer than on the flat at 25+.)

If you are negotiating an 'Alp worth' of switchbacks on a heavy bike with narrow rims there is a problem of heat. You can cook the brake blocks for them to suddenly have the stopping power of cheese on toast, maybe to send you out wide on a bend (if you are lucky). In rescuing your bike from the vegetation you may touch one of the rims for it to be too hot to handle. Although it is obvious that brakes get hot it is not necessarily an obvious consideration whilst you are otherwise doing a good job of descending.

Broken bones and flesh wounds can be fixed by the doctors and nurses, what you want to avoid is traumatic brain injury where your brain smacks on the inside of the skull, bruises, swells up and leads to brain damage. With age brain damage becomes less sustainable in that you cannot recover from it in the way that those that suffer strokes recover brain function. Allegedly just falling over onto a hard surface can give your head a 15 mph whack, enough to ruin your brain for maybe a lifetime. A helmet will not change this sudden deceleration force significantly - your brain behaves much like a person not wearing a seatbelt in a car where crumple zones in the bonnet don't really help. Technically standing still is therefore 'dangerous' - you could faint and die, just like that.

'Feeling safe' has nothing to do with this technicality, but hey, if you are in the zone, on a cool road with no mechanical/traffic/weather/surface problems then 65 mph 'feels' fantastic and if you make it to the bottom in one piece then '65' is obviously safe...

Perhaps a more logical way for a UK cyclist to look at the situation is the 'twenty is plenty' road safety campaign. This campaign is all about pedestrians and setting traffic speeds that are survivable for them. Getting hit by a car at 20mph as a pedestrian is probably similar to falling off a bike at 20 mph - a lot more survivable than 30 mph.

I think the max speed I've ever reached is about 60mph. However, since then I've had at least three experiences where I was riding downhill at a somewhat lower speed (30-40mph) and encountered a rough section of road and nearly lost control. A bike that seems perfectly stable on smooth pavement can get surprisingly unstable at moderately high speed on rougher pavement.

So I have a self-imposed speed limit of about 30mph.

Certainly more skilled riders on well-tuned bikes may go faster with reasonable safety, but I doubt that any rider is particularly safe above 40mph or so, except on a closed track or at least on a hill the rider is familiar with.